Sources of alternating current ("AC") electrical power may be adversely affected by attached electrical loads which distort the phase and shape of the AC power waveform. Dynamic loads imposed, for example, by rotating electrical equipment and in particular by motor controller power supplies, can change the AC waveform from its normal sine wave shape.
Distortion of the AC power waveform can be quantified by spectral analysis. A pure AC sine wave at a line frequency (60 Hz in the United States, 50 Hz in Europe) will exhibit only a single spectral component at that line frequency, whereas a distorted waveform will exhibit a number of spectral components at higher frequencies. These higher spectral components describe the amplitude of a series of sine waves whose sum produces the distorted waveform shape.
For distortion of the AC waveform that remains constant from cycle to cycle, the frequencies of the spectral components will be a harmonic or integer multiple of the fundamental AC waveform line frequency. For example if the line frequency is 60 Hz, the distortion will cause spectral components at the first, second, and third harmonics (120 hertz, 180 hertz, 240 hertz) and so on. Current standards for AC waveform fidelity require measurement of the amplitudes of harmonics of the fundamental frequency up to the 41st harmonic. An accuracy of within 1% of the amplitude of the fundamental frequency is also required.
Because the harmonic content of the AC line voltage can vary rapidly, it is desired that such measurements be made for relatively short lengths of waveform data, for example, in as little as one cycle. Unfortunately, measurements of the harmonic components on such short durations of waveform data has proven relatively inaccurate.